Avoid listening port collision by letting OS assing ports

Previously, we could in tests potentially run into listening port
collisions resulting into `InvalidSocketAddress` errors. These errors
could surface if we rolled port numbers that either collided with other
concurrent tests *or* with other unrelated services running on
localhost.

Here, we simply let the OS assign us a free port number when setting up
the testing nodes, which avoids such collisions altoghether (mod the
potential TOCTOU race here, which we ignore for now).

Author: tnull

tests/common/mod.rs

@@ -215,22 +215,18 @@ pub(crate) fn random_storage_path() -> PathBuf {
 	temp_path
 }
 
-pub(crate) fn random_port() -> u16 {
-	let mut rng = rng();
-	rng.random_range(5000..32768)
-}
-
 pub(crate) fn random_listening_addresses() -> Vec<SocketAddress> {
 	let num_addresses = 2;
-	let mut listening_addresses = Vec::with_capacity(num_addresses);
+	let mut listening_addresses = HashSet::new();
 
-	for _ in 0..num_addresses {
-		let rand_port = random_port();
+	while listening_addresses.len() < num_addresses {
+		let socket = std::net::TcpListener::bind("127.0.0.1:0").unwrap();
+		let rand_port = socket.local_addr().unwrap().port();
 		let address: SocketAddress = format!("127.0.0.1:{}", rand_port).parse().unwrap();
-		listening_addresses.push(address);
+		listening_addresses.insert(address);
 	}
 
-	listening_addresses
+	listening_addresses.into_iter().collect()
 }
 
 pub(crate) fn random_node_alias() -> Option<NodeAlias> {